Ejector driven steam generator start up system

ABSTRACT

A generating system is presented that includes a steam generator, a separator coupled to the generator, a supply providing feed water, a start-up system coupled to and receiving the feed water, and a recirculation system coupled to the start-up system. The steam generator operates in a plurality of operating modes. In one mode the steam generator generates a flow of steam and fluid. The separator separates the flow into components of steam and fluid. The recirculation system receives the feed water from the start-up system and provides a required flow to the steam generator during at least one mode. The recirculation system includes an ejector. The ejector induces a portion of the fluid from the separator into the recirculation system, mixes the induced fluid with the feed water to provide a recirculation flow, and the recirculation system provides the required flow to the steam generator including the recirculation flow.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority benefit under 35 U.S.C. §119(e) of copending, U.S. Provisional Patent Application Ser. No. 61/166,045, filed Apr. 2, 2009. The disclosure of the aforementioned U.S. patent application is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The present disclosure relates generally to a boiler system and, more particularly, to a boiler system including a steam generator having a start up system that re-circulates fluid back to the steam generator with an ejector.

2. Related Art

Generally speaking, start-up of a steam generating system is a stepped process as steam from the generator is not immediately available. Similarly, during low load conditions, steam may not be sufficiently available. Typically, a start-up system is incorporated into the steam generating system to protect components of the generating system during these low steam conditions, e.g., at start-up and low load conditions. For example, the start-up system protects water walls, which include a plurality of tubes, of a steam generator from overheating when the steam generator is below a minimum once-through load by providing an additional flow of feed water. The minimum once-through load is a load (approximately twenty to about fifty percent (20%-50%) of the full load flow) where the flow (e.g., a required flow) through the water walls is enough to protect the tubes from overheating without the need for the additional flow. As shown in FIG. 1, a conventional steam generating system 10 includes a start-up system 20 that provides the additional flow during low load or start up conditions. During start-up or low load conditions, a feed water pump 22 of the start-up system 20 pumps feed water 24 to a heater 26 to heat the water. Heated feed water 28 is then passed to a steam generator 30 and, in particular, to water walls of the steam generator 30, to protect the water walls from damages caused if the tubes overheat. Water and/or steam are passed from the generator 30 to a separator 40. The separator 40 separates the steam and water. The steam is passed to, for example, a turbine, while the water flows through a conduit 50 and a valve 62 to a flash tank 60. At the flash tank 60 the water exits the system 10 through a drain 70.

As is generally known, substantially all of the feed water 24 and 28 that is not converted into steam within the water walls is drained off by the start up system 20 and replaced by the feed water 24 flow. This operating mode is referred to herein as a drain mode. The steam generator 30 has to fire at a rate sufficient to bring the feed water up to saturation temperature and then to generate steam. Initially during the start up of the steam generator, none of the feed water is converted to steam and all the feed water provided to the generator must be drained off (e.g., in the drain mode). As steam is generated, less water has to be drained off until the boiler reaches the once-through mode where substantially all of the feed water flow is converted into steam. In the once-through mode, the flow of feed water for steam generation is referred to herein as a required flow. Until the once-through mode load is attained, a minimum flow of feed water is sent to the generator for water wall cooling and, as noted above, is referred to herein as the additional flow. Accordingly, prior to attaining the once-through mode load, both the required flow and the additional flow are provided to the steam generator.

As noted above, when operating in a once-through mode, the steam generator converts substantially all of the feed water into steam. Therefore, during the once-through operating mode, there is substantially no flow in the conduit 50 from the separator 40 to the flash tank 60. During operational loads below the once-through load, for example, at loads below a minimum once through load, which typically ranges from about twenty percent (20%) to about fifty percent (50%) of a full load flow, the additional flow is required for water wall cooling. During these low load conditions, a flash tank valve 62 modulates to allow flow to the flash tank 60 thus allowing the feed water pump to deliver the minimum required flow for cooling.

The inventors have recognized that this method of cooling the water walls of steam generators during start-up and low load conditions is inefficient and wastes water and thermal energy, which in turn translates into increased cost to operate the steam generation system. Accordingly, a need exists for a reliable and economical method of cooling components of a steam generator during low load conditions.

SUMMARY OF THE INVENTION

A steam generating system includes a steam generator, a separator coupled to the steam generator, a feed water supply providing feed water, a start-up system coupled to and receiving the feed water from the feed water supply, and a recirculation system coupled to the start-up system. The steam generator operates in a plurality of operating modes. In at least one of the operating modes the steam generator generates a flow of steam and fluid. The separator receives the steam and fluid flow and separates components of steam and fluid from the steam and fluid flow. The recirculation system receives the feed water from the start-up system and provides a required flow to the steam generator during at least one of the plurality of operating modes. The recirculation system includes an ejector. The ejector induces at least a portion of the fluid from the separator into the recirculation system, mixes the induced fluid with the feed water to provide a recirculation flow, and the recirculation system provides the required flow to the steam generator including the recirculation flow.

In one embodiment, the recirculation system further includes a bypass valve and a block valve. The bypass valve and the block valve are selectively operated during the operating modes of the steam generator to provide one or both of the required flow and the recirculation flow. For example, when the steam generator is operating in a once-through operating mode, the bypass valve opens and the block valve closes to isolate the ejector and to provide the feed water from the feed water pump through the recirculation system to the steam generator as the required flow. In a low load condition operating mode of the steam generator, the bypass valve selectively restricts or prohibits flow and the block valve opens to permit the ejector to induce the fluid from the separator, mix the induced fluid and the feed water (e.g., to provide the recirculation flow), and provides the required flow to the steam generator including the recirculation flow for steam generation and to cool the water wall. In one embodiment, the low load condition includes a start-up operating mode of the steam generator. In another embodiment, the low load condition includes operating modes below about twenty percent (20%) to about fifty percent (50%) of a full load flow of the steam generator.

In one embodiment, the recirculation system includes a throttling valve coupled to the ejector. The throttling valve receives the mixture of the induced fluid and the feed water (e.g., the additional flow) from the ejector and provides the required flow to the steam generator including the recirculation flow. The throttling valve controls the feed water pressure and flow through the ejector as loads vary during ejector operation.

In another embodiment, the steam generating system further includes a downcomer conduit coupling the separator and the recirculation system. The recirculation system further includes a check valve that provides one way flow from the downcomer conduit to the ejector and thus prohibits reverse flow back to the downcomer conduit.

In still another embodiment, the steam generating further includes a flash tank coupled to the downcomer conduit through a flash tank valve. When the steam generator is operating in a once-through operating mode, there is no fluid flowing from the downcomer conduit to the flash tank and nothing is drained from the system.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the Figures, which are exemplary embodiments, and wherein the like elements are numbered alike.

FIG. 1 is a schematic diagram of a conventional steam generating system having a start up system.

FIG. 2 is a schematic view of a once-through steam generator having a start up system including a system for recirculation of liquid.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 2 illustrates a steam generating system 100 including a start-up system 120, a recirculation system 200, a steam generator 130, and a separator 140. The start-up system 120 and the recirculation system 200 cooperate to provide a required flow 132, including an additional flow to protect the components of the generator (e.g., water walls) from damage caused by overheating, to the steam generator 130 during operation. As described herein, during start-up and/or low load operating conditions, a recirculation flow 270 mixes with the heated feed water flow and/or replaces the feed water flow to provide the required flow 132 to the steam generator 130. For example, during start-up and low load conditions, a feed water pump 122 pumps feed water 124 to a heater 126 for heating. Heated feed water 128 is then passed to the recirculation system 200. In the recirculation system 200 the heated feed water 128 selectively passes directly to the steam generator 130 through a bypass valve 210 as a heated feed water flow 128A, passes to an ejector 240 through a block valve 230 as a heated feed water flow 128B, or passes through both circuits/legs of the recirculation system 200 proportionally as feed water flows 128A and 128B. Accordingly, the bypass valve 210 and the block valve 230 regulate flow within the recirculation system 200. In one embodiment, the by-pass valve 210 may operate in, for example, a fully open mode permitting complete flow there through, a partially closed mode permitting a portion of a flow (e.g., a restricted flow) there through, and a fully closed mode permitting no flow there through. When the steam generator 130 is in the once-through mode, the block valve 230 isolates the ejector 240 and the heated feed water 128A is provided directly to the steam generator 130 by the bypass valve 210 (e.g., in the fully open mode) as the required flow 132. In start-up and/or low load conditions, the bypass valve 210 selectively regulates the heated feed water flow 128A by, for example, cycling though one or more of the aforementioned modes to shut-off flow or is at least partially close to restrict a portion of the feed water flow 128A that mixes with the recirculation flow 270 from the ejector 240. Flow within the recirculation system 200 is described in detail below in relation to various operating modes of the steam generating system 100.

Initially, in a start-up mode, the bypass valve 210 opens to receive the heated feed water 128 and pass the heated feed water 128A as the required flow 132 (including the additional flow) to the steam generator 130 and, in particular, to water walls of the steam generator 130, to protect the water walls from damages caused when tubes of the water walls overheat. As with the steam generating system 10 of FIG. 1, a flow 134 (e.g., water and/or steam) is passed from the steam generator 130 to the separator 140 over a separator supply line 142. The separator 140 separates the flow 134 into, for example, steam and a fluid (e.g., water). The steam is passed through a conduit 144 to, for example, a turbine, and the fluid is output into a conduit 150 such as, for example, a downcomer line 150. In one embodiment, the ejector 240 induces fluid (or a percentage/portion thereof) from the downcomer 150 to the recirculation system 200 through a check valve 260 as a recirculation fluid 180. The check valve 260 prevents reverse flow of the recirculation fluid 180 from the recirculation system 200 into the downcomer line 150. In one embodiment, similar to the steam generating system 10 of FIG. 1, the downcomer line 150 is also coupled to a flash tank 160 through a flash tank valve 162. Once at the flash tank 160, the fluid exits the system 100 through a drain 170.

At the ejector 240, the recirculation fluid 180 mixes with the heated feed water 128B pass through the block valve 230 to form the recirculation flow 270 passed through the throttling valve 250. The throttling valve 250 controls the feed water pressure and flow as the load varies when the ejector 240 is in operation. That is, the throttling valve 250 provides control for optimal differential pressure and improved ejector performance. During start-up and/or low load conditions, the recirculation flow 270 mixes with the heated feed water flow 128A from the bypass valve 210. The portion of heated feed water 128A passed through the bypass valve 210 is controlled based on, for example, a volume of the recirculation flow 270. For example, the bypass valve 210 is operated in one or more of the aforementioned modes (e.g., the fully open mode, the partially closed mode, and the fully closed mode) to regulate the flow 132 to the steam generator 130.

As can be appreciated, at start-up and low load conditions, all, none or a percentage of the fluid 180 (e.g., water) flowing from the separator 140 may be selectively recirculated to the steam generator 130 within the required flow 132. At a predetermined time period or occurrence of a desired event such as, for example, attainment of the once-through mode, the fluid 180 (if any) is provided to the flash tank 160 where it may be drained off or stored. In one embodiment, at once-through mode of the steam generator 130, the recirculation system 200 is controlled to directly pass the heated feed water 128A through the bypass valve 210 as the flow 132 to the steam generator 130 and to inhibit the heated feed water flow 128B to the ejector 240 and throttling valve 250 by the block valve 230. Similarly, the recirculation system 200 is controlled via the bypass valve 210 and the block valve 230 to respond to low load operation (e.g., below the minimum once-through flow rate equal to a range of about twenty percent (20%) to about fifty percent (50%) of the full load flow) to maintain a minimum rate of the required flow 132 to the water walls.

The recirculation system 200 and the ejector 240 selectively recirculate a percentage of the recirculation fluid 180 (e.g., the recirculation flow 270) to increase a temperature of the required flow 132 (including the additional flow) entering the steam generator 130. The heat recovery realized by the recirculation process reduces an amount of fuel that, for example, the steam generator 130 consumes to heat the feed water 124 during the start-up process and low load conditions. The recirculation of fluid from the separator 140 back to the steam generator 130 also reduces a loss of feed water 128 as less water needs to be drained off. As can also be appreciated, a savings in terms of the feed water 124, 128 supplied also yields a saving in the cost of chemical treatment of the feed water and thus operational savings and greater efficiencies in operating the system 100. An additional benefit in the recirculation system 200 as described herein is seen to be reduced operational costs as ejector systems, such as the ejector 240, have no moving parts, proven reliability, easy operation and maintenance. Additionally, it should be appreciated that should the ejector 240 be disabled or require maintenance, the generating system 100 can be started and operated in the drain mode (as in FIG. 1) thus there is no loss of operability due to the incorporation of the recirculation system 200. In one embodiment, shut-off valves (not shown) are disposed about various components of the steam generating system 100 to selectively prohibit flow to the components. For example, in one embodiment, shut-off valves are disposed in line with each of the inputs and output of the ejector to shut off flow to the ejector 240. As noted above, when the ejector 240 is isolated, the recirculation system 200 may operate to direct flow through the bypass valve 210 such that the steam generating system 100 operates in the drain mode.

At least some features of the steam generating system 100 including the start-up system 120 and the recirculation system 200 as described herein, include the addition of the ejector 240 and the throttling valve 250. For example, the throttling valve 250 allows the feed water pump 122 to operate at a higher pressure than the steam generator 130 and to deliver the motive force for the ejector 240. In one embodiment, using the head provided by the feed water pump 122, the ejector 240 induces recirculation of the fluid in the downcomer 150 from the separator 140 back through to the steam generator 130. Additionally, since the ejector 240 has no moving parts, the recovery of heat through the recirculation system 200 gives rise to only minimal concern for additional maintenance and/or repair.

Moreover, as described above, conventional systems (such as the steam generating system 10 of FIG. 1) discharge all separated water through the flash tank, losing both heat and water. Use of an ejector driven recirculation system 200 with the start-up system 120 (as described herein) allows recovery of much of the heat and fluid, thereby saving costs in several ways. Less heat input is required to service the steam generator 130 and develop steam as the incoming water temperature is increased by mixing the heated feed water 128 with the recirculation fluid 180. Feed water supply and treatment costs are reduced since less water is flashed off to steam and lost. The proposed circuit has minimal maintenance requirements, thus redundant equipment and associated systems are not required.

While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 

1. A steam generating system, comprising: a steam generator operating in a plurality of operating modes, in at least one of the plurality of operating modes the steam generator generating a flow of steam and fluid; a feed water supply providing feed water; a start-up system coupled to and receiving the feed water from the feed water supply; and a recirculation system coupled to the start-up system, the recirculation system receiving the feed water from the start-up system and providing a required flow to the steam generator during at least one of the plurality of operating modes, the recirculation system includes an ejector, the ejector induces at least a portion of the fluid from the steam generator into the recirculation system, the ejector mixing the induced fluid with a portion of the feed water to provide a recirculation flow, and the recirculation system providing the required flow to the steam generator including the recirculation flow.
 2. The steam generating system of claim 1, wherein the recirculation system further includes a bypass valve and a block valve, the bypass valve and the block valve being selectively operated during the plurality of operating modes of the steam generator to provide the required flow and the recirculation flow to the steam generator.
 3. The steam generating system of claim 2, wherein when the steam generator is operating in a once-through operating mode, the bypass valve opens and the block valve closes to isolate the ejector and to provide the feed water from the recirculation system to the steam generator as the required flow.
 4. The steam generating system of claim 2, wherein when in a low load condition operating mode, the block valve opens and the bypass valve is selectively controlled such that the ejector induces the fluid from the steam generator, mixes the induced fluid with the feed water to form the recirculation flow and the recirculation system provides the required flow from the recirculation system to the steam generator including the recirculation flow.
 5. The steam generating system of claim 4, wherein the low load condition includes a start-up operating mode of the steam generator.
 6. The steam generating system of claim 4, wherein the low load condition includes operating modes below a minimum once-through load equal to a range of about twenty percent (20%) to about fifty percent (50%) of a full load flow of the steam generator.
 7. The steam generating system of claim 1, wherein the recirculation system further includes a throttling valve coupled to the ejector, the throttling valve receives the mixture of the induced fluid and the feed water and provides the mixture as the recirculation flow, the throttling valve controls the feed water pressure and flow through the ejector as loads vary during ejector operation.
 8. The steam generating system of claim 1, wherein at least one of the plurality operating modes of the steam generator includes a drain mode, the drain mode including isolating the ejector and operating the recirculation system such that the required flow to the steam generator is provided by the feed water from the feed water supply.
 9. A steam generating system, comprising: a steam generator operating in a plurality of operating modes, in at least one of the plurality of operating modes the steam generator generating a flow of steam and fluid; a separator coupled to the steam generator, the separator receiving the steam and fluid flow and separating components of steam and fluid from the steam and fluid flow; a feed water supply providing feed water; a start-up system coupled to and receiving the feed water from the feed water supply; and a recirculation system coupled to the start-up system, the recirculation system receiving the feed water from the start-up system and providing a required flow to the steam generator during at least one of the plurality of operating modes, the recirculation system includes an ejector, the ejector induces at least a portion of the fluid from the separator into the recirculation system, the ejector mixing the induced fluid with the feed water to provide a recirculation flow, and the recirculation system providing the required flow to the steam generator including the recirculation flow.
 10. The steam generating system of claim 9, wherein the recirculation system further includes a bypass valve and a block valve, the bypass valve and the block valve selectively operated during the plurality of operating modes of the steam generator to provide the required flow and the recirculation flow to the steam generator.
 11. The steam generating system of claim 10, wherein when the steam generator is operating in a once-through operating mode, the bypass valve opens and the block valve closes to isolate the ejector and to provide the feed water from the recirculation system to the steam generator as the required flow.
 12. The steam generating system of claim 10, wherein when in a low load condition operating mode, the block valve opens and the bypass valve is selectively controlled such that the ejector induces the fluid from the steam generator, mixes the induced fluid with the feed water to form the recirculation flow and the recirculation system provides the required flow from the recirculation system to the steam generator including the recirculation flow.
 13. The steam generating system of claim 12, wherein the low load condition includes a start-up operating mode of the steam generator.
 14. The steam generating system of claim 12, wherein the low load condition includes operating modes below a minimum once-through load equal to a range of about twenty percent (20%) to about fifty percent (50%) of a full load flow of the steam generator.
 15. The steam generating system of claim 9, wherein the recirculation system further includes a throttling valve coupled to the ejector, the throttling valve receives the mixture of the induced fluid and the feed water and provides the mixture as the recirculation flow, the throttling valve controls the feed water pressure and flow through the ejector as loads vary during ejector operation.
 16. The steam generating system of claim 9, further including a downcomer conduit coupling the separator and the recirculation system, and wherein the recirculation system further includes a check valve providing one way flow from the downcomer conduit to the ejector.
 17. The steam generating system of claim 16, further including a flash tank coupled to the downcomer conduit through a flash tank valve, wherein when the steam generator is operating in a once-through operating mode, no fluid returns from the downcomer conduit and no flow is provided to the flash tank.
 18. The steam generating system of claim 9, wherein at least one of the plurality operating modes of the steam generator includes a drain mode, the drain mode including isolating the ejector and operating the recirculation system such that the required flow to the steam generator is provided by the feed water from the feed water supply. 